This is a homework problem ... obviously ... that will involve (e.g.) the use of the Perl functions splice and join.   Take a good look at the two pages I’ve hyperlinked, and I’ll bet that you will be able to solve this assignment for yourself in a very little while.

And if you can’t, then your instructor needs to know, and to be the one to help you, so that s/he will also be aware of the points that might need further clarification in an upcoming classroom session.   (If you are having this problem, then you are not the only one of your classmates to be doing so.)   Put the instructor into “the loop.”